Last week, the Kansas City Council voted to increase its local minimum wage to $13 by 2020, handing yet another victory to labor activists who have been campaigning to lift worker pay across the country. However, the ordinance included a big exception: The new minimum won't apply to employees under 18. It's an adults-only raise.

As Lydia DePillis noted at Wonkblog, that move was quite controversial. Many saw it as watering down the law and worried it would hurt teens who support families...